1. Technical Field
The present invention relates to a method for generating hydrogen by water splitting. The present invention also relates to a photoelectrochemical cell and a semiconductor electrode used for the method.
2. Description of the Related Art
United States Patent Application Pre-Grand Publication No. 2012/0028141, which is equivalent to WO 2011/016244, discloses a photoelectrochemical cell 900. As shown in FIG. 9, this photoelectrochemical cell 900 includes: a semiconductor electrode 920 including a substrate 921, a first n-type semiconductor layer 922 disposed on the substrate 921, and a second n-type semiconductor layer 923 and a conductor 924 disposed apart from each other on the first n-type semiconductor layer 922; a counter electrode 930 connected electrically to the conductor 924; an electrolyte 940 in contact with surfaces of the second n-type semiconductor layer 923 and the counter electrode 930; and a container 910 accommodating the semiconductor electrode 920, the counter electrode 930 and the electrolyte 940. In the semiconductor electrode 920, relative to a vacuum level, (I) band edge levels of a conduction band and a valence band in the second n-type semiconductor layer 923, respectively, are higher than band edge levels of a conduction band and a valence band in the first n-type semiconductor layer 922, (II) a Fermi level of the first n-type semiconductor layer 922 is higher than a Fermi level of the second n-type semiconductor layer 923, and (III) a Fermi level of the conductor 924 is higher than the Fermi level of the first n-type semiconductor layer 922. The photoelectrochemical cell 900 generates hydrogen by irradiation of the second n-type semiconductor layer 923 with light.